1. Field of the Invention
The present invention relates to a DC-AC converter (hereinafter, referred to as inverter) which generates an AC voltage for driving a load from a DC power source such as a power source adapter provided in an electric apparatus, or a battery, and a controller IC for such an inverter.
2. Description of the Related Art
A cold cathode fluorescent lamp (CCFL) has been used as a backlight source for a liquid crystal monitor of a notebook computer or a liquid crystal display device of an LCD television receiver. A CCFL has an efficiency and a long life which are approximately equivalent to those achieved by a usual hot cathode fluorescent lamp, however omitted is a filament that is used in a hot cathode fluorescent lamp.
In order to start and operate a CCFL, a high AC voltage is required. For example, the starting voltage is about 1,000 V (root-square-mean value, hereinafter this is applicable to an AC voltage), and the operating voltage is about 600 V. These high AC voltages are generated by a DC power source of a notebook computer or an LCD television receiver in which an inverter is used.
In general, a Royer circuit has been used for an inverter for a CCFL. The Royer circuit is configured by a saturable core transformer, control transistors, etc. which enables the circuit to self-oscillate by the nonlinear permeability of the saturable core transformer and the nonlinear current gain characteristics of the control transistors. For this reason, the Royer circuit itself does not require an external clock or a driver circuit.
However, a Royer circuit is basically a constant voltage inverter, which cannot maintain the constant output voltage when the input voltage or the load current is varied. Therefore, a regulator for supplying a power to a Royer circuit is required. Because of this requirement, downsizing of an inverter using a Royer circuit is difficult and its power conversion efficiency is relatively low.
As disclosed in JP Hei 10-50489, an inverter for a CCFL which is configured so as to enhance the power conversion efficiency is proposed. In the inverter, a first semiconductor switch is connected in series to a primary winding of a transformer, a series connection of a second semiconductor switch and a capacitor is connected in parallel to the primary winding of the transformer, and a coupling capacitor and a load are connected in series to a secondary winding of the transformer. The primary current of the transformer is fed back to a controlling circuit to be compared with a reference voltage, thereby forming a control signal. The first and second semiconductor switches are ON/OFF-controlled by the control signal, so that a predetermined AC power is supplied to the load.
A full bridge (H bridge) type inverter used for a CCFL where four semiconductor switches are used is proposed in U.S. Pat. No. 6,259,615. In the inverter, an output terminal of an H bridge is connected in series via a capacitor for oscillation to a primary winding of a transformer, and a load is connected to a secondary winding of the transformer. Among four semiconductor switches constituting the H bridge, a first set of two semiconductor switches form a current path in a first direction in the primary winding of the transformer, and a second set of two semiconductor switches form a current path in a second direction in the primary winding of the transformer. A current flowing through the secondary winding of the transformer is fed back to a controlling circuit to be compared with a reference voltage, thereby generating a control signal in which pulses have a fixed same width and relative positions of the pulses are controlled. The control signal is supplied to the semiconductor switches of the H bridge to adjust the power supply to the load. The voltage of the secondary winding of the transformer is detected in order to perform over-voltage protection.
Also a configuration is known in which a current flowing through a CCFL is detected, the duty of pulse width modulation (PWM) of lighting/non-lighting in an intermittent operation of an inverter power source apparatus is controlled to adjust the time ratio of lighting/non-lighting so that the currents has a predetermined value as disclosed in JP 2002-221701.
[Problems that the Invention is to Solve]
In the inverters such as disclosed in said patent publications, the ON times of the semiconductor switches are controlled so that the current of a predetermined value flows through the load, thereby controlling the power supply to the load. In order to reduce the power supply to the load, the width of the control pulse for turning ON a semiconductor switch is narrowed. However, the technique of stably supplying a small power to a load while narrowing the width of the control pulse has limitations. Therefore, the lighting control range of a CCFL serving as a load is hardly widened in the direction of the lower limit.
In the inverter of Patent literature 3, the time ratio of lighting (ON)/non-lighting (OFF) in an intermittent operation is controlled. However, it is difficult to finely perform a light control due to only the control of the intermittent operation.